KDP (KH2PO4, or monopotassium phosphate, or potassium dihydrogen phosphate) s a water soluble crystal known for its non-linear optical properties.
KDP crystals are used for producing electro-optical elements (Pockels cells), waveplates, frequency converters (second, third and fourth harmonic generators for autocorrelators and cross-correlators, parametric generators and amplifiers) in various pulse lasers